Ultra-thin wafer level stack packaging method

ABSTRACT

A method of forming an ultra-thin wafer level stack package and structure thereof are provided. The method includes providing a first wafer having a plurality of base chips thereon, selectively binding the first wafer to a second substrate, lapping the first wafer to reduce its thickness, dicing the lapped first wafer, bonding a plurality stack chips to each base chip and packaging the base chip with the bonded stack chips to form an IC package. Thus, each IC package comprises at least a base chip and a stack chip. The IC package has a size almost identical to the base chip and a thickness a little larger than the combined thickness of the base chip and the stack chip. If a known good die inspection of the base chips and stack chips are carried out prior to wafer level packaging, overall yield of the IC package is increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 93102569, filed on Feb. 5, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor chip packagingtechnology. More particularly, the present invention relates to a methodof forming a stack package and structure thereof.

2. Description of the Related Art

In recent years, the technology developments and the popularity ofportable, handheld and consumer electronic products has almostovershadowed the conventional personal computer (PC) products. Tofacilitate the manufacturing of these electronic products, most devicesare designed towards higher storage capacity and smaller line width toincrease the packing density, the operational frequency, to reduce thepower consumption and to achieve the integration of multi-functions. Inthe packaging technology of the integrated circuits (ICs), the chipscale package (CSP) and the wafer level package have been invented tomeet the requirements for higher input/output pin count, higherheat-dissipating capacity and reduction of the package size.Furthermore, associated packaging techniques for reducing the weight andcost are also being developed.

In the development of the chip scale package (CSP), a variety oftechniques such as the single chip package, the stack chip package andthe planar multi-chip package (MCM) are developed. The aforementionedtechniques can reduce the dimension of a package to a size only slightlylarger than the original size of the chip. However, the technology ofthe stack chip packages and the planar multi-chip packages must becombined with the known good die (KGD) technique to produce a highyield.

Unlike the conventional chip scale package (CSP) method, the waver levelpackage or wafer level chip scale package (WL-CSP) method packages anentire wafer before dicing up the wafer. Hence, the WL-CSP method caneliminate many process steps such as underfilling, assembling, substrateprocessing, chip attaching and wire bonding so that the overallfabrication cost can be substantially reduced. In general, the wafer canbe packaged regardless of the size of the chip or the pin count. Inother words, the wafer level packaging is able to reduce the processsteps to thereby shorten the fabrication cycle time, to improve theperformance and to lower down the cost. In addition, the amount ofsaving increases correspondingly with the size of the wafer. Therefore,the wafer level packaging method is particularly advantageous to thewafer processing plants shifting from 8-inch wafer production to 12-inchwafer production.

System on chip (SOC) and system in a package (SIP) are regarded as twoprincipal techniques for producing miniaturized and multi-functionalsemiconductor devices in the future. In particular, the system on chip(SOC) technique has some promising applications in manufacturing digitalinformation products. At present, the multi-chip package modules withhigh operating frequency, low cost, small size and short fabricationcycle are the dominant packaging type. For example, a drawing chip or amemory chip is often fabricated by the multi-chip package technology toachieve the high processing frequency, super-fast processing speed andthe capacity of integration of multi-functions. Therefore, the knowngood die technique is important in the packaging process of themulti-chip package technology. After a number of chips are packaged, andthe electrical properties of each packaged chip is tested. The chipsthat fail the test are immediately discarded and the chips that pass thetest are integrated by attaching to a packaging product. In this way,the area of the printed circuit board of the package system is reducedand the yield of a conventional multi-chip package is increased.

SUMMARY OF THE INVENTION

Accordingly, at least one object of the present invention is to providea method of forming an ultra-thin wafer level stack package capable ofsimplifying the packaging process and increasing overall yield andthroughput of the package.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described herein, thepresent invention provides a method of forming an ultra-thin wafer levelstack package and package structure thereof. The package structure ofthe present invention comprises an independent chip set. The independentchip set is obtained by dicing an selectively adhered wafer. Theselectively adhered wafer comprises a first wafer and a second substrateattached to the first wafer by using a plurality of adhesives, whereinthe second substrate may comprise a plurality of chips, and thepositions of the chips are matched with the first wafer. The first waferhas a plurality of base chips formed thereon. The first wafer separatesfrom the second substrate by a distance equal to the thickness of theadhesive glue layer. Accordingly, each independent chip set comprises abase chip and a portion of the second substrate. In addition, a methodof forming the adhesives may comprise, for example, a dispensing method.Moreover, a method of forming the adhesives may comprise forming adouble side tape having a region comprising the adhesives. Thereafter,after the selectively adhered wafer is obtained and before theindependent chip sets are formed, a thermal curing step may further beperformed for curing the adhesives.

The method of forming an ultra-thin wafer level stack packages,providing a first wafer having a plurality of base chips thereon, and asecond substrate. Next, a first surface of the first wafer is bonded toa first surface of the second substrate by using a plurality ofadhesives to form an selectively adhered wafer, wherein a distancebetween the first wafer and the second substrate is equal to a thicknessof the adhesives. Next, a plurality of independent chip sets are formed,wherein each of the independent chip sets comprises the base chip and aportion of the second substrate. The independent chip set is formed bycutting a second surface of the first wafer of the selectively adheredwafer and cutting a second surface of the second substrate of theselectively adhered wafer. In addition, a method of cutting theselectively adhered wafer may comprise a diamond blade cutting method ora laser cutting method. Finally, the independent chip sets are packagedto achieve a packaged IC or chip.

In a preferred embodiment of the present invention, a process ofdetecting a known good die (KGD) of the base chips of the first wafer iscarried out before the commencement of the packaging.

In another preferred embodiment of the present invention, one or morestacked chips are bonded to the base chip after the step of forming theindependent chip sets but before the step of packaging the independentchip sets.

In another preferred embodiment of the present invention, the secondsurface of the first wafer is polished after the step of forming theselectively adhered wafer but before the step of cutting the secondsurface of the first wafer of the selectively adhered wafer.

According to an aspect of the present invention, at least a stack chipis also attached to the base chip of each independent chip set.

According to another aspect of the present invention, the first waferhas a thickness between 200 μm to 500 μm.

According to another aspect of the present invention, a polishingprocess is performed over the surface of the first wafer after producingthe selectively adhered wafer but before dicing a surface of the firstwafer in the selectively adhered wafer.

According to another aspect of the present invention, the first waferhas a thickness between 30 μm to 250 μm after polishing the first waferin the selectively adhered wafer.

According to another aspect of the present invention, the first waferpreferably has a thickness between 30 μm to 80 μm after polishing thefirst wafer in the selectively adhered wafer.

According to another aspect of the present invention, a ‘known good die’(KGD) inspection of the stack chip or base chip is performed.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIGS. 1 to 8 are top views and cross-sectional views illustrating aprocess of forming an ultra-thin wafer level stack package and packagestructure according to the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

FIGS. 1 to 8 are top views and cross-sectional views illustrating theprocess of forming an ultra-thin wafer level stack package according toa preferred embodiment of the present invention. First, referring toFIG. 1, a first wafer 102 having a plurality of base chips 112 thereonis provided. As shown, an area marked by the intersection of a solidhorizontal line and a solid vertical line defines each base chip 112. Asecond substrate 104 is also provided, wherein the second substrate 104may comprise no chip, or one or more chips previously formed thereon,and the size and position of the chips previously formed has been presetto match with the first wafer 102. The second substrate 104 can be atransparent or a non-transparent glass substrate comprised of, forexample, but not limited to, a silicon wafer substrate, a plasticsubstrate, an acrylic substrate or a polymer substrate. Thereafter, thesecond substrate 104 is adhered to the surface of the first wafer 102with the base chips 112 through a plurality of adhesives 106 to form aselectively adhered wafer 108. Referring to FIG. 1, only a few specificareas on the first wafer 102 are adhered to the adhesive 106. The firstwafer 102 separates from the second substrate 104 by a distance equal tothe thickness of the adhesives 106. In one embodiment of the presentinvention, the method of forming the adhesives 106 comprises, forexample, dispensing method. In another embodiment of the presentinvention, a specific double side tape, for example, having a shape thesame as the first wafer 102 may be provided. In addition, the adhesives106 may be disposed on a portion of the specific double side adhesivetape, for example, the portion corresponding to the first wafer 102.Therefore, after the first wafer 102 is adhered to the second substrate104 by the adhesives 106, a thermal curing process may further beperformed for curing the adhesives 106.

In one preferred embodiment of this invention, a known good die (KGD)inspection of the base chips 112 on the first wafer 102 is carried outprior to the commencement of the packaging process.

After attaching the first wafer 102 to the second substrate 104 usingthe adhesives 106, the exposed surface of the first wafer 102 of theselectively adhered wafer 108 facing the direction indicated by thecutting direction 222 in FIG. 2 is polished. The original thickness ofthe first wafer 102 is in a range of about 200 μm to about 500 μm. Afterthe polishing process, the first wafer 102 of the selectively adheredwafer 108 has a thickness in a range of about 30 μm to about 250 μm.Preferably, the thickness of the first wafer 102 after the polishingprocess is in a range of about 30 μm to about 80 μm.

Referring to FIG. 2, the first wafer 102 of the selectively adheredwafer 108 is polished before proceeding with the following steps.However, it is to be noted that even if the polishing step is skipped,the following process steps are the same.

Next, the first wafer 102 of the selectively adhered wafer 108 is dicedalong the direction indicated by the cutting direction 222. The depth ofthe cut is greater than the thickness of the polished first wafer 102but smaller than the total thickness of the first wafer 102 and theadhesive 106. Hence, the saw in the dicing process is prevented fromcutting into the second substrate 104. Thereafter, the structure asshown in FIG. 3A and FIG. 3B may be obtained. In one embodiment of thepresent invention, the method of dicing the selectively adhered wafer108 may comprise a diamond blade cutting method or a laser cuttingmethod.

After the dicing process, the first wafer 102 is cut into a plurality offirst base chips 302 as shown in FIGS. 3A and 3B. Thereafter, the secondsubstrate 104 is diced along the direction indicated by the cuttingdirection 324 in FIGS. 3A and 3B. The depth of the cut is greater thanthe thickness of the second substrate 104 but smaller than the totalthickness of the second substrate 104 and the adhesive 106. Thus, thesaw in the dicing process is prevented from cutting into the first basechips 302. Referring to FIGS. 4A and 4B, after the second substrate 104is diced up, the selectively adhered wafer 108 is cut into a pluralityof independent chip sets, wherein each independent chip set comprisingat least a first base chip 302, an adhesive 106 and a portion of thesecond substrate 404.

In a preferred embodiment of the invention, as shown in FIG. 5A, afterthe second substrate 104 is diced up along the cutting direction asshown in FIGS. 3A and 4A, an external stack chip 502 may be bound to thesurface of the first base chip 302 of each or some of the independentchip sets. Thereafter, the whole structure as shown in FIG. 5A may bewired and packaged, wherein every packaged integrated circuits (IC) maycomprise, the first base chip 302 and the stack chip 502 stacked on thefirst base chip 302. In one preferred embodiment of the presentinvention, a known good die (KGD) inspection of the stack chip 502 canbe performed before the stack chip 502 is bound to the independent chipset. In addition, the thickness of the IC package is substantiallysimilar to the whole thickness of the second substrate 404 and the firstbase chip 302.

As shown in FIG. 6A, after binding the stack chip 502 to the first basechip 302 of the independent chip set, the independent chip set may bewired and packaged to form an integrated circuit (IC) package that atleast comprises a first base chip 302, a stack chip 502 stacked on thefirst base chip 302, and one or more chips 522 on the cut secondsubstrate 404. In addition, the thickness of the IC is substantiallysimilar to the whole thickness of the second substrate 404 and the firstbase chip 302.

In an alternative embodiment as shown in FIG. 5B, a plurality of stackchips may bind to the surface of the first base chip 302 of theindependent chip sets after the second substrate 104 is diced up. Here,only two stack chips 512 and 514 are shown, however, more than two stackchips can be provided in the present invention. In one preferredembodiment of this invention, a known good die (KGD) inspection of thestack chips 512 and 514 can be carried out before the stack chips arebound to the independent chip set. In addition, the thickness of the ICis substantially similar to the whole thickness of the second substrate404 and the first base chip 302.

After binding the stack chips 512 and 514 to the first base chip 302 ofthe independent chip set, the excess portion of the second substrate 404is removed to form a second base plate 604 as shown in FIG. 6B.Thereafter, the independent chip set is packaged to form an integratedcircuit (IC) package at least comprising a first base chip 302, aplurality of stack chips 512 and 514, and one or more chips 522 on thecut second substrate 404. In addition, the thickness of the IC packageis substantially similar to the whole thickness of the second substrate404 and the first base chip 302.

In another embodiment as shown in FIGS. 7 and 8, a plurality ofindependent chip sets may be obtained after the surface of the secondwafer 104 of the selectively adhered wafer is diced along the directionsshown in FIGS. 3B and 4B. In every independent chip set, the overlappedregion between the second substrate 404 and the bas chip 302 may bedependent on the demand of the process. In addition, whether the chips522, the stack chips 512 and 514 are disposed on the second substrate404 may be dependent on the demand of the process.

In summary, the present invention provides an ultra-thin wafer levelstack packaging structure as shown in FIGS. 7 and 8. The packagestructure comprises a base chip 302, a second base plate 604 and a stackchip 502 or a plurality of stack chips 512 and 514. The base chip 302has a plurality of areas with the second base plate 604 bonded to onearea of the base chip 302 by an adhesive 106, and with the stack chip502 or the stack chips 512, 514 bonded to another area of the base chip302. In addition, one or more chips 522 may be disposed on the secondsubstrate 404. Moreover, the thickness of the IC package issubstantially similar to the whole thickness of the chip 522, the secondsubstrate 404 and the base chip 302.

This invention provides a method of forming an ultra-thin wafer levelstack package. The advantages of the invention is that the selectivelyadhering of a second substrate to a first wafer having a plurality ofbase chips thereon can provide the reduction of the thickness of thefirst wafer, wherein the second substrate may be pre-designed to matchwith the first wafer. The thickness of the second substrate need not betoo thick but need to be thick enough to maintain the selectivelyadhered wafer of the first wafer and prevent the second substrate fromany deformation during the dicing process of the first wafer. Thus, forexample, the second substrate may have a thickness close to that of aconventional wafer. By dicing the first wafer of the selectively adheredwafer, the problem caused from cutting a thin polished first wafer canbe avoided. After binding a stack chip to the base chip, the entireassembly is packaged to form an integrated circuit (IC) package. The ICpackage may comprise at least a base chip and one or more stack chips,wherein the size of the IC package is substantially similar to the wholesize of the base chip and the stack chip(s). In addition, the packagehas a size similar to the base chip and a thickness close to thecombined thickness of the second substrat, the polished base chip andthe adhesive. In other words, the package has a thickness close to aconventional wafer. Therefore, the package method of the presentinvention may provide a stack chip package structure.

Furthermore, through the packaging method of the present invention,chips fabricated by different processes can be integrated to form asingle package. The circuits that can be fabricated by the sameprocessing steps can be fabricated on the same chip to shorten thefabrication cycle thereof. In addition, chips manufactured by differentprocesses may be packaged together, therefore, the packaged structure islight, thin, and small. Furthermore, if a ‘known good die’ inspection isincorporated to check the stack chips or the base chips, the yield willbe improved significantly and the cost will be lowered considerably.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method of forming a wafer level stack package, comprising:providing a first wafer having a first surface and opposing secondsurface and having a plurality of base chips thereon; providing a secondsubstrate, the second substrate having a first surface and opposingsecond surface and a plurality of chips wherein the chips of the secondsubstrate is matched with respect to the position and size of the basechips of the first wafer; selectively binding a portion of the firstsurface of the first wafer to a portion of the first surface of thesecond substrate by using a plurality of adhesives to form, aselectively adhered first wafer wherein a distance between the firstsurface of the first wafer and the first surface of the second substrateis equal to a thickness of the adhesives; forming a plurality ofindependent chip sets, wherein each of the independent chip setscomprises the base chip and the corresponding chip of the secondsubstrate, an order of steps of forming the independent chip setscomprise: a first cutting at the second surface of the first wafer ofthe selectively adhered first wafer, wherein a depth of the firstcutting is greater than a thickness of the first wafer but smaller thanthe total thickness of the first wafer and the adhesive; and a secondcutting at the second surface of the second substrate of the selectivelyadhered first wafer to form the independent chip sets, wherein a depthof the second cutting is greater than the thickness of the secondsubstrate but smaller than the total thickness of the second substrateand the adhesive; and packaging the independent chip sets.
 2. The methodof claim 1, wherein a good die (KGD) inspection of the base chips of thefirst wafer is performed before the commencement of the packaging. 3.The method of claim 1, further comprising a step of binding a stack chipto the base chip of the independent chip sets after the step of formingthe independent chip sets but before the step of packaging theindependent chip sets.
 4. The method of claim 3, wherein a good die(KGD) inspection of the stack chip is performed before the step ofbinding the stack chip to the base chip of the independent chip sets. 5.The method of claim 1, further comprising a step of binding a stack chipto each of the base chip of the independent chip sets after the step offorming the independent chip sets but before the step of packaging theindependent chip sets.
 6. The method of claim 5, wherein a good die(KGD) inspection of each of the stack chip is performed before the stepof binding the stack chips to each one of the base chips of theindependent chip sets.
 7. The method of claim 1, wherein a thickness ofthe first wafer is in a range of about 200 μm to about 500 μm.
 8. Themethod of claim 1, further comprising a step of polishing the secondsurface of the first wafer is performed after the step of forming theselectively adhered first wafer but before the step of cutting thesecond surface of the first wafer.
 9. The method of claim 8, wherein athickness of the first wafer is in a range of about 30 μm to about 250μm after the second surface of the first wafer of the selectivelyadhered first wafer is polished.
 10. The method of claim 8, wherein athickness of the first wafer is in a range of about 30 μm to about 80 μmafter the second surface of the first wafer of the selectively adheredfirst wafer is polished.
 11. The method of claim 1, wherein the secondsubstrate comprises a plurality of chips, and a position and size ofeach of the chips is matched with those of the base chips of the firstwafer.
 12. The method of claim 1, wherein a method of forming theadhesives comprise a dispensing method.
 13. The method of claim 1,wherein a method of forming the adhesives comprises forming a doubleside tape having a region comprising the adhesives.
 14. The method ofclaim 1, wherein after the selectively adhered first wafer is obtainedand before the independent chip sets are formed, further comprisesperforming a thermal curing step for curing the adhesives.
 15. Themethod of claim 1, wherein a method of cutting the selectively adheredfirst wafer comprises a diamond blade cutting method or a laser cuttingmethod.
 16. The method of claim 1, wherein the second substratecomprises a glass substrate.
 17. The method of claim 1, wherein thesecond substrate comprises a silicon wafer substrate.
 18. The method ofclaim 1, wherein the second substrate comprises a plastic substrate. 19.The method of claim 1, wherein the second substrate comprises an acrylicsubstrate.
 20. The method of claim 1, wherein the second substratecomprises a polymer substrate.
 21. The method of claim 1, wherein thesecond substrate is fabricated using a transparent material.
 22. Themethod of claim 1, wherein the second substrate is fabricated using anon-transparent material.